A 40 MHz cavity has been designed and built at CERN as part of the preparation of the PS as injector for LHC. The cavity will provide the necessary bunch spacing of 25 ns prior to injection into SPS and subsequently LHC. The mechanical design of the copper coated steel cavity was dominated by space constraints in the PS tunnel and by vacuum requirements. The salient design features described are i) tight, multipactor-free, capacitive coupling from the power amplifier, ii) fast RF feedback, iii) inductively coupled tuners, iv) an efficient, pneumatically operated gap short-circuit. The operation cycle consists of an adiabatic capture up to 100 kV gap voltage, a non-adiabatic jump to 300 kV, and subsequent bunch rotation. The multipactor voltage level at the gap lies below the operating voltage range and is easily passed through. A fast RF feedback system with a total group delay of 220 ns copes with heavy beam loading (1011 protons/bunch) and prevents unwanted interaction with other beams in the PS. The cavity has recently been installed, the nominal gap voltage of 300 kV has been attained, and bunch lengths below 8 ns have been achieved in first tests at nominal intensity. Experimental results are reported
